1. Technical Field
This invention is concerned with a pad assembly for use in a disc brake.
2. Related Art
Disc brakes are well known and operate by bringing pads into engagement with opposite sides of a disc to bring about deceleration of the disc. Thus, a typical disc brake has at least one pad mounted on each side of the disc and means for moving the pads towards the disc. The term “pad assembly” is used herein to mean an arrangement with one or more pads mounted on a common carrier plate.
In order to improve the engagement between the pads and the disc and to reduce uneven wear of the pads and/or the disc, some disc brakes, for example for use on railway vehicles, provide that the pads are mounted on the carrier plate in such a way that they can make limited movement relative to the carrier plate to enable them to “conform” better to the disc. Specifically, the pad may be mounted so that it can tilt, pivot or rock to bring its disc-engaging surface parallel to the surface of the disc. Where there are a plurality of pads, they may be mounted so that they can move towards or away from a carrier plate so that the pads all engage the disc. Examples can be found in U.S. Pat. No. 5,934,418 and German utility model number 2982113.
Another example is described in EP 1318321A which discloses a pad assembly in which pads are mounted on the carrier plate by means of a headed fastener. Each pad comprises a block of friction material mounted on a backing plate. The headed fastener is fixed to the carrier plate and passes through a hole in the backing plate of the pad and the head is on the opposite side of the backing plate from the carrier plate so that the head prevents the pad from becoming detached from the carrier plate. The backing plate is a relatively loose fit on the headed fastener so that the pad can make limited movements relative to the carrier plate. The limited movements can be a rocking, tilting or pivoting movements or may be linear movements towards the carrier plate or may be a combination of such movements. The limited relative movements enable the pad to conform more accurately to the surface of the disc. These relative movements take place against the action of spring means which is provided by resilient tongues projecting from the carrier plate and engaging in shallow recesses formed in the backing plate. The tongues are pre-tensioned against the backing plate by the headed fastener.
In the pad assembly which is disclosed in EP 1318321, the braking force, which is applied by the disc to each pad during braking, is transmitted by the backing plate of the pad to the resilient tongues of the carrier plate. This arrangement is unsatisfactory because in certain circumstances very considerable stress is applied to the tongues which can cause cracking at their junction with the remainder of the carrier plate.
A further, more complex arrangement shown in EP0263752 discloses an arrangement whereby a backing plate on which a number of brake pads are mounted is subsequently mounted adjacent a carrier plate by means of a substantially central spring-biased mounting. Additionally, there is provided a pair of bores, one in the backing plate, and one in the carrier plate which can be aligned to allow for an indexing pin to be received in both to prevent rotation of the backing plate relative to the carrier plate. However, it is clear from this document that the central spring-biased mounting carries substantially all the load when the brake pads engage against the friction surface to give rise to a braking effect, as is the case in all other prior art carrier plate/backing plate arrangements, and that the provision of an indexing pin is merely to prevent relative rotation.